Jurors in the trial of a Cairns mechanic accused of murdering a couple over a car have been sent home after their first day of deliberations.

Brandon Peter MacGowan, 43, is accused of shooting Scott Maitland in the head and stabbing Cindy Masonwells after failing to fix a panel van they had paid him $14,000 to restore in 2012.

Summing up, Justice Jim Henry told the Supreme Court jury the case was essentially a "whodunnit" and the identity of the couple's killer was "hotly contested".

He urged the jury, which began deliberating after lunch, to be cautious in considering lies told by MacGowan in determining his guilt or innocence.

MacGowan told the court he last saw Mr Maitland and Ms Masonwells when they stole his customer's van and his lawyers said they might have been killed by an unknown third party.

The accused has admitted he lied to police but has denied murdering the couple.

The couple had arrived in Cairns to attend a friend's party and pick up the panel van they had paid the accused to restore.

Justice Henry said the jury could theoretically return manslaughter verdicts, but commented the manner in which the couple was killed and their bodies being dumped in remote bushland could point to the killer intending to murder or do grievous bodily harm to the pair.

He said this was not a formal direction but something the jury could consider.

The judge also said the prosecution's case was circumstantial as there were no eyewitnesses to the killings and MacGowan had denied involvement.

He said if the jury were to find MacGowan guilty, that verdict "should be the only rational inference".

He said they had to rule out competing theories pointing to MacGowan's innocence or acquit him.

The jury is due to resume deliberations on Friday.

Accused's defence scenario 'ridiculous'

In his closing address, prosecutor Todd Fuller said MacGowan's contention that he last saw the couple when they drove off in anger in his customer's vehicle was a fantasy.

Mr Fuller told the court that MacGowan had resorted to numerous lies to cover up the fact he had squandered the money paid to him and had not fixed the van.

With pressure mounting, MacGowan felt he had no other option but to kill the pair, Mr Fuller said.

"He put himself under pressure by his inactivity, by his squandering of their money, and the day of reckoning was coming and his only response to that was ... extreme violence," he said.

"He took advantage of his relationship with these two people when they were in a vulnerable position and he killed them."

Mr Fuller told the court the alleged killings were the acts of a "calculated man".

He said if MacGowan's story was true, he was a very unlucky man.

"They've then been killed by somebody who has turned up at Fearnley Street and put on a bandanna and called themselves "Brando", then dumped their bodies and put the van back in a carpark in the street over from where [MacGowan] works, and snuck back in and hung the keys up on the board in his office," he said.

"The word ridiculous doesn't even describe that scenario."

Mr Fuller said MacGowan's DNA and Ms Masonwells' blood were found on a hose reel.

He said in the aftermath of alleged killings, MacGowan tried to say the couple had driven off in their green panel van, but it had poor tyres and the Cardwell range was dangerous.

"That is an attempt at a sculpted tale and that's because he knows what he's done," Mr Fuller said.

Lies did not make accused a murderer, defence says

However, defence barrister Joshua Trevino argued the dispute over the $14,000 debt was not a credible motive and pointed to weaknesses in the crown's forensic and witness evidence.

"Doesn't such a motive seem like a pretty unbelievable reason to kill two people?" Mr Trevino said.

"It would be an understatement to say that would an extreme way to deal with a civil debt, a civil dispute."

Mr Trevino said while MacGowan had told many lies, it did not make him a murderer.

"He's a big-note, he's a blowhard, but that doesn't make him a murderer," he said.

The jury is likely to retire to consider its verdict later today after Justice Henry finishes summing up the case.